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Old 13th June 2008, 04:43 PM   #1
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Default Amp bridging and performance question

Hi all. I have several 3 way loudspeaker designs I'm working on and both are actively crossed W to TM (possibly some passive components for mild response shaping though) so I will be biamping. The woofer in one design is the B&C 15NW76 and I would like and amp that is capable of taking this driver near it's power limits of 500w continous.

After a lot of thought I settled on getting three Parasound A23's and run one to the pair of TM's and run the other two bridged mono to each W. I've already got one A23 bought and on the way, but now I'm wondering if I should just use one A21 for the woofers? The A23 bridged mono is 400w @ 8 ohm and the A21 is 2x250w @8 ohm. Now the difference is only about 2dB in the real world (maybe less after power compression), but there is the aspect of more headroom that is appealing. Anyway I guess my main quesion is: does other performance (distortion, etc.) suffer when bridging an amp? Would be better off giving up the 2 dB in return for slightly better SQ?
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Old 14th June 2008, 07:10 PM   #2
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No one has any insight on this?
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Old 14th June 2008, 09:34 PM   #3
jcx is offline jcx  United States
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within thier I-V capability the amps shouldn't be within orders of magnitude of the distortion of the sub itself driven with 100s of Watts

the shortest path to lower distortion would be to add more subs

more radiating area = lower excursion, distortion and multiple sources can improve room mode issues
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Old 14th June 2008, 10:12 PM   #4
sek is offline sek  Germany
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In general, a bridged amp can work and sound as good as a regular (single channel). Two things are of main importance: bridging circuitry (i.e. how the symmetry between the input stages is maintained) and power supply (i.e. how it can drive the lower impedance load of each channel).

With the 15NW76 (good choice, IMO) you get a lot of potential dynamic capability. B&C actually states 1.2kW (how do you arrive at 500W?) of dynamic power capability! That's way more than the A23 could possibly deliver, bridged or not. So in that sense I would say: don't bother, the extra 3dB (how do you arrive at 2dB?) aren't worth the hassle, you're wasting 6dB headroom on top of that anyway!

With an A21 however, it really makes a difference WTR dynamic performance. Notably, a modern high power neodymium woofer can take nearly any power beating as long as the Xmax isn't exceeded. It's just the long term average (i.e. over the course of multiple seconds) that let thermal power compression set in.

All in all, my opinion is as follows: the difference between A23 and A21 will be more than just barely noticeable with the 15NW76, but choosing the higher power amp is only mandatory for use in high SPL use anyway. So if you don't want to crank it from time to time, don't bother.

On another note: I don't want to question the choice of Parasound amps, but they spec 60A peak with a 1.2kW toroid for the A21. This appears to be a suboptimal choice for the B&C unit: it needs large voltage peaks, not so much current. A calculation example indicates a requirement of 1.200W = 98V * 12.25A for single channel mode or 2 * 600W = 2 * (49V * 12.25A) for bridged mono mode. Parasound specs 750W continuous power with >1.5dB headroom. Let's make this a generous 2dB, resulting in about 950W peak. This resolves to 87V max. output voltage instead of the optimal 98V as of the calculation above. But while there may be other amplifiers with more dynamic headroom (crest factor), the build quality and the honesty in specifications and documentation are worth a thumbs up to Parasound. Almost professional.


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within thier I-V capability the amps shouldn't be within orders of magnitude of the distortion of the sub itself
Yeah, but not maxing out the power capacity of the sub. And connecting more woofers in parallel isn't recommended due to the 4 Ohm limit of the Parasound amps (i.e. no 2 Ohm specification, see manual page 15). Correct me if I'm wrong.

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Old 14th June 2008, 10:40 PM   #5
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Quote:
Originally posted by sek

With the 15NW76 (good choice, IMO) you get a lot of potential dynamic capability. B&C actually states 1.2kW (how do you arrive at 500W?) of dynamic power capability! That's way more than the A23 could possibly deliver, bridged or not. So in that sense I would say: don't bother, the extra 3dB (how do you arrive at 2dB?) aren't worth the hassle, you're wasting 6dB headroom on top of that anyway!
I was just using the Nominal power handling spec, although looking at it the Continous power spec (1000w) is much more realistic with music. The Nominal spec is pretty rigorous I would say! The A23 bridged will deliver 400w and the A21 stereo will deliver 250w, so roughly 2dB difference.

Quote:
Originally posted by sek
All in all, my opinion is as follows: the difference between A23 and A21 will be more than just barely noticeable with the 15NW76, but choosing the higher power amp is only mandatory for use in high SPL use anyway. So if you don't want to crank it from time to time, don't bother.
But I will, thus the conundrum

Quote:
Originally posted by sek
On another note: I don't want to question the choice of Parasound amps, but they spec 60A peak with a 1.2kW toroid for the A21. This appears to be a suboptimal choice for the B&C unit: it needs large voltage peaks, not so much current. A calculation example indicates a requirement of 1.200W = 98V * 12.25A for single channel mode or 2 * 600W = 2 * (49V * 12.25A) for bridged mono mode. Parasound specs 750W continuous power with >1.5dB headroom. Let's make this a generous 2dB, resulting in about 950W peak. This resolves to 87V max. output voltage instead of the optimal 98V as of the calculation above. But while there may be other amplifiers with more dynamic headroom (crest factor), the build quality and the honesty in specifications and documentation are worth a thumbs up to Parasound. Almost professional.
Interesting. How did you determine the voltage requirement of 98v for the 15NW76? Now I'll either be powering each woofer with one bridged A23 (400w) or one channel of an A21 (250w), from that perspective which has the greater crest factor in those modes?

Thanks for the help, this is exactly what I needed to know!
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Old 14th June 2008, 11:24 PM   #6
jcx is offline jcx  United States
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I'd claim the Parasound's are wasted on dedicated woofer drive, any high feedback amp is going to be good at a few 100 Hz, for the price difference doubling the woofers (which doubles radiating efficency too) and getting say ep2500 which will easily drive the parallel load, in fact they're cheap enough for 2 ep2500 with one channel per ea of 4 drivers and still cost less than one Parasound

puting the 2 more drivers into separate sub boxes and dedicating them to the bottom couple of octaves is another way of reducing distortion
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Old 15th June 2008, 12:23 AM   #7
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Quote:
Originally posted by jcx
I'd claim the Parasound's are wasted on dedicated woofer drive, any high feedback amp is going to be good at a few 100 Hz, for the price difference doubling the woofers (which doubles radiating efficency too) and getting say ep2500 which will easily drive the parallel load, in fact they're cheap enough for 2 ep2500 with one channel per ea of 4 drivers and still cost less than one Parasound

puting the 2 more drivers into separate sub boxes and dedicating them to the bottom couple of octaves is another way of reducing distortion

Oh I'd considered the EP2500. But these woofers will be run fairly high, if not in this design then another one which will be a 2 way. From everything I've heard about the EP2500, nobody has been particularly happy using these to run loudspeakers in home, only for live sound and subwoofers. I wish it were otherwise, they are certainly a lot of watts for a good price.

My design allows for only one 15" woofer per loudspeaker, in a 3 way OB dipole arrangement. They may be crossed to monopole subs depending on how the baffle works out and whether I use a shelving filter. A lot still to be determined there.
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Old 15th June 2008, 12:35 AM   #8
sek is offline sek  Germany
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Quote:
Originally posted by augerpro
I was just using the Nominal power handling spec
That's why I'm asking. I see a spec of 600W nominal, 1.200W peak on the 15NW76 product page. Are we talking about the same speaker?


Quote:
How did you determine the voltage requirement of 98v for the 15NW76?
It was not so much meant as a requirement than more as a capability. Here's how:

Take a look at the datasheet. B&C give an impedance curve that shows 8Ohms minimum between 200Hz and 300Hz. The same curve also shows above 10Ohms below 100Hz, although the impedance in the low frequency range strongly depends on your enclosure and alignment. So unless you've got measurements of your speaker in your enclosure, let's assume 8Ohms for safety reasons.

B&C also specify 1.200W dynamic power handling. Watt's law (P=V*I), combined with Ohm's law (V=R*I), can be written as P = I^2 * R or P = V^2 / R, solving for V as V = sqrt(P * R) and for I as I = sqrt(P / R) (with sqrt being the square root function).

With P:=1.200W and R:=8Ohm, the voltage now turns out to be V = sqrt(1.200W * 8Ohm) ≈ 98V.

But remember this is only the safe capability considering an impedance of 8 Ohm. According to the B&C data, the speaker can take ≈110V below 100Hz, ≈155V below 70Hz, and so on. That's why such speakers often survive when connected to presumably oversized amplifiers, they don't neccessarily consume all the current an amp could deliver, but happily take the voltage until their excursion limit is exceeded. When such speakers fail due to overload on strong amps, they rarely burn. They usually break.


Quote:
Now I'll either be powering each woofer with one bridged A23 (400w) or one channel of an A21 (250w), from that perspective which has the greater crest factor in those modes?
Crest factor is a measure for the headroom the amp has left under load. Parasound specs the two identical WTR headroom: '>1.5dB'.

As of your question, 400W is more power than 250W, so it's safe to say that the bridged A23 will offer more dynamic range.

But that's of not too much value for you in this case. Both amps will work, but none of the two will use the potential of the 15NW76.


Quote:
Originally posted by jcx
I'd claim the Parasound's are wasted on dedicated woofer drive
[...]
getting say ep2500 which will easily drive the parallel load
That's also what I'm thinking, although I wouldn't recommend anything with an ear on it for quality applications.

A good quality professional amplifier with a slow/quiet fan assembly will do. The A23 lists at 850$. A QSC PLX 3102 (max. two 15NW76 per channel, so 4 total per amp) also retails for under 1.000$, an also suitable 1802 (one 15NW76 per channel only, so 2 total per amp) goes for under your 850$.


Quote:
Originally posted by augerpro
nobody has been particularly happy using these to run loudspeakers in home
A lot of people do in HT, some in music setups. They had to fiddle with the cooling fans, though.

Cheers,
Sebastian.
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Old 15th June 2008, 05:23 PM   #9
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Quote:
Originally posted by sek


That's why I'm asking. I see a spec of 600W nominal, 1.200W peak on the 15NW76 product page. Are we talking about the same speaker?
Bah, I kept clicking the 12NW76 to look at specs, but you are correct it's the 15NW76 I'll be using.


Quote:
Originally posted by sek
But that's of not too much value for you in this case. Both amps will work, but none of the two will use the potential of the 15NW76.
Very true. I need to recheck the enclosure model, but I think it reaches xmax at 35hz with 500w input power, being an open baffle. So I don't need an amp that can take the driver all the way to it's limits, but something that is close and sounds good.


Quote:
Originally posted by sek
A good quality professional amplifier with a slow/quiet fan assembly will do. The A23 lists at 850$. A QSC PLX 3102 (max. two 15NW76 per channel, so 4 total per amp) also retails for under 1.000$, an also suitable 1802 (one 15NW76 per channel only, so 2 total per amp) goes for under your 850$.
So these would have suitable sound quality? Remember there may also be a 2 way design using these drivers and they will cross up around 1khz or so. I know QSC makes great amps for pro audio and the subwoofer guys love them, but I've never seen anyone use them for in home hifi type applications. I'll look into this farther.

Thank you everyone for the help so far!




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Old 15th June 2008, 06:01 PM   #10
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QSC was only picked as a recommendation, the PLX2 series simply delivers among the highest quality bang for the buck in the Parasound's price range. But QSC certainly isn't the only option.

The trouble is usually in the circuitry surrounding the actual amplifier circuit: input signal processing, limiting, protection circuitry. In the 500W+ class you just need these features, because if something goes wrong, the rest of your equipment at least gets a slight chance of survival.

And this circuitry (as well as the cooling arrangement) usually compromises sound quality in pro amplifiers. Wether they are worse than the Parasound Halo series you can only find out by testing: find a rental company nearby and get a pro unit (whatever your choice from their gear) over the weekend. Should go for below 50 bucks.
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